The use of transdermal patches for the delivery of various drug systems has met with increasing success in the pharmaceutical industry, particularly in view of specific problems which have arisen in connection with drugs taken by other means, and because of their implications in terms of long term application of drugs in a particular simple manner. One of the specific problems which has been encountered in connection with the use of various drugs has been the ability to apply a drug in a simple system which employs the drug in admixture with an adhesive base system for application to the skin, or non-adhesive base system having an outer drug permeable adhesive layer. The ability to do this with various types of drugs can be impeded by various considerations, such as differences in viscosity, solubility, therapeutic drug delivery rate, drug migration within the system, and the like.
Simple monolithic transdermal systems incorporate their active agents, i.e., drugs, directly into a single pressure sensitive adhesive layer. These systems have the advantage of being thin, elegant, and relatively easy to manufacture, but must compromise between optimizing the adhesive matrix for drug delivery versus its ability to adhere to the skin.
The known “double-disk” transdermal patch uses a larger auxiliary patch over a smaller active agent delivery patch to improve or ensure adhesion to the skin. The adhesive matrixes of the inner and outer patches can be independently optimized for active agent delivery and adhesion, respectively. When the inner and outer patches are laminated together to form the completed system, their adhesive matrixes come into direct contact and begin to equilibrate. As the systems equilibrate, time-dependent changes occur such as the loss of active agents from the inner patch and the simultaneous accumulation of active agents in the outer patch. This phenomenon can alter the performance of the transdermal patch if any of the components in the inner patch, especially those that are needed to achieve or sustain active agent delivery, have appreciable affinity for the outer patch adhesive matrix. Moreover, this effect will become more profound with time until equilibrium is achieved.
One solution in preventing the equilibrium of the two adhesive matrixes is to maintain their physical separation, and not to allow the adhesives to come into direct contact with each other during storage. Following application to the skin, these adhesive matrixes will be in direct contact, but the equilibrium process, typically two-three years, is slow compared to the transdermal delivery process, generally less than seven days. However, in the double-disk transdermal patch, the circumferential edge of the inner patch containing the active agent is exposed to the overlying outer patch and its adhesive matrix. This structure of the double-disk transdermal patch allows for the circumferential migration of active agent from the inner patch into the adhesive matrix of the outer patch.